1. Field of the Invention
The present invention relates to a vehicle braking device comprising: a master cylinder in which a master piston having a rear surface facing a booster fluid pressure working chamber is slidably housed in a casing; pressure regulating valve means for regulating output fluid pressure from a fluid pressure generation source to fluid pressure corresponding to a brake operation input from a brake operation member to cause the fluid pressure to act on the booster fluid pressure working chamber; a backup piston that has a closed front end facing the booster fluid pressure working chamber and is slidably housed in the casing so as to directly press the master piston from behind corresponding to an operation of the brake operation member when the output fluid pressure from the fluid pressure generation source is reduced; a short cylindrical separator fluid-tightly fitted and secured to the casing so that a front outer periphery of the backup piston is slidably fitted to the short cylindrical separator; and a cylindrical sleeve that forms a channel between the cylindrical sleeve and the separator, forms an annular passage communicating with the channel between the cylindrical sleeve and an outer periphery of the backup position, and is fluid-tightly fitted and secured to the casing backward of the separator, annular seal members for sealing front and rear sides of the annular passage being mounted between the separator and the backup piston, and between the sleeve and the backup piston.
2. Description of the Related Art
Such a vehicle braking device has been known, for example, from Japanese Patent Application Laid-open No. 4-283157, in which a seal member for sealing a front side of an annular passage is mounted to an outer periphery of a backup piston so as to come into repulsive contact with an inner periphery of a separator, and a seal member for sealing a rear side of the annular passage is mounted to an inner periphery of the sleeve so as to come into repulsive contact with the outer periphery of the backup piston.
In the conventional vehicle braking device, however, the seal member is mounted to the outer periphery of the backup piston so as to seal the front side of the annular passage, and thus an axial length of the separator has to be set relatively long in order to secure a portion with which the seal member that advances along with the backup piston comes into slide contact when the backup piston advances so as to directly press the master piston, thereby relatively increasing an axial length of a casing to increase the size of the vehicle braking device.